Electronic displays or monitors can be fabricated using different display technologies, such as cathode-ray tube (CRT), electroluminescent displays (ELD), light-emitting diode displays (LED), liquid crystal displays (LCD), and plasma display panel (PDP).
In CRT technology, electron beams scan across a display surface line by line in order to provide each pixel data to the display surface. The display data is thus represented by the pixels via the electron beam current. Modulation of the electron beam current varies the characteristics of the displayed image.
In electroluminescence, a material, called electroluminescent material, emits light in response to an electric current or an electric field. Electroluminescence is the result of the recombination of electrons and holes in the electroluminescent material where the excited particles release their energy as photons. An electroluminescent display can be constructed by sandwiching a layer of electroluminescent material such as GaAs between two layers of conductors. Each conductor layer has parallel electrode strips running perpendicular to each other. One layer must be transparent in order to emit light. At each intersection is a pixel emitting visible light when a current flows.
In LCD, each pixel consists of a liquid crystal blocking or unblocking incoming light in response to the electric field between the electrodes. In the absence of an electric field, the liquid crystal molecules are arranged to allow the light to pass through, and the pixel appears transparent. When a voltage is applied across the electrodes, the liquid crystal molecules are distorted, reducing the passing light, and the pixel appears gray. For higher voltage, the liquid crystal molecules are completely distorted and the pixel will appear black. By controlling the voltage applied across the liquid crystal layer in each pixel, passing light can be controlled which illuminates the pixel correspondingly.
In PDP, each pixel comprises an ionizable gas such as neon or xenon. When an electric field is applied across the electrodes, the gas ionizes to form a plasma and as the ions accelerating toward and colliding with the electrodes, photons are emitted. By controlling the voltage applied across the ionized gas in each pixel, generated light from each pixel can be controlled.
Generally, electronic displays can be classified into two major types. One includes light emitter displays where the pixels emit photons (CRT, ELD, LED, PDP) and one includes light modulator displays where the pixels allow the passage of light (LCD). In light emitter displays, the maximum display brightness is controlled by the current or voltage applied to the individual pixels, subjected to their material and physical limitations.
In light modulator displays, the classification can further include displays using reflective light where light is reflected back toward the viewer after passing through the modulated pixels. Displays may also be transmissive, wherein light is radiated toward the viewer after passing through the modulated pixels. Other displays may also be transflexive, a combination of reflective and transmissive with two sources of light, one to reflect and one to radiate toward the viewer. In light modulator displays, the maximum display brightness is controlled by the light sources, and the individual modulated elements control the perceived brightness of a pixel.
Display systems are judged by many metrics, including horizontal and vertical resolution, brightness, color purity, display size, frame rate, and image artifacts. Some of these characteristics are more important than the others, depending on the customers, and sometimes simply because they are compared directly while on display in a store.
Brightness, or maximum brightness of a display, is one important characteristic for display systems since bright images are a general consumers' preference. The brightness of an image on an electronic display is characterized by luminance measured in luminous intensity (candela) per unit area (cd/m2=1 nit). The brightness of CRT or PDP can be controlled by varying the current or voltage to the display, while for LCDs, by varying the intensity of the light sources. However, this simple brightness control can introduce image artifacts of contrast fidelity, or color washed out.
Image contrast in a display is another important attribute. Maximum image contrast describes the achievable light intensity difference in the image between the brightest and dimmest pixels. Contrast is also affected by ambient illumination, since it is also added to the display intensity of the displayed image. However, contrast fidelity, the ability to provide contrast approaching that of natural pictures, is the criterion for a best image display, and maximum contrast merely provides the adjustment capability to achieve contrast fidelity.
Thus consistent maximum brightness is a big concern for electronic display manufacturing, since not all displays have the same maximum brightness given the variations in materials, process equipment, manufacturing process, and operation parameters. Further, with contract manufacturing and OEM (original equipment manufacturer) services, maximum brightness might be very noticeable, especially when viewing next to each other as in a display showcase.